Machines such as wheel loaders, excavators, dozers, motor graders, and other types of heavy equipment use multiple actuators supplied with hydraulic fluid from one or more pumps on the machine to accomplish a variety of tasks. These actuators are typically velocity controlled based on, among other things, an actuation position of an operator interface device. In particular, when an operator moves a particular interface device to a specific displaced position, the operator expects a corresponding hydraulic actuator to move at a predetermined velocity in a desired direction. This predetermined velocity and associated fluid flow into the actuator required to produce the velocity are, however, generally fixed within permanent relationship maps during testing of a similar test machine at a manufacturing facility, and may not account for machine-to-machine variability. Accordingly, every machine may behave somewhat differently when actuated in the same manner by the same operator. If left unchecked, this variability could cause reduced machine control, performance, and efficiency.
One attempt to reduce the effects of machine-to-machine variability in the control of a position-in, velocity-out hydraulic system is disclosed in U.S. Pat. No. 6,775,974 that issued to Tabor on Aug. 17, 2004 (the '974 patent). In particular, the '974 patent describes a hydraulic system having a joystick movable by an operator to produce an electrical signal indicative of a direction and a desired rate at which a corresponding hydraulic actuator is to move. The hydraulic system also has a pressure sensor configured to sense a system pressure at an electro-hydraulic proportional valve associated with the hydraulic actuator, and a controller in communication with the joystick, the pressure sensor, and the electro-hydraulic proportional valve. The controller is configured to request a desired velocity for the hydraulic actuator based on the electrical signal, and determine varying forces acting on the hydraulic actuator based on a signal from the pressure sensor. The controller is further configured to determine a unique valve flow coefficient, which characterizes fluid flow through the particular electro-hydraulic proportional valve, that is required to achieve the desired velocity. Activation of the electro-hydraulic valve is then performed based on the valve flow coefficient.
Although the system of the '974 patent may be potentially helpful in reducing machine-to-machine variability, it may still be less than optimal and lack applicability. In particular, the system of the '974 patent may fail to consider system delays inherent to pump and cylinder response, as well as valve behavior during cylinder movement. In addition, the system may lack applicability to machines where pressure variations at the valve do not substantially affect flow through the valve.
The disclosed hydraulic control system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.